All comes down to sales; as mentioned earlier, there was a HUGE drop in sales from 70 to 71 for Barracuda and 'Cuda.
E Bodies never sold particularly well, even with the decent first year sales. Changes in 71 were likely planned before the 70s went on sale, so this was a planned update and when sales #s fell so sharply, they backed off, made the cars more plain to attempt to appeal to a wider audience and simply kept building them til the stampings wore out.

See Pontiac Aztek for a similar track, albeit 30 years later (they deleted body side moldings and updated a few features, but basically then just built out the car until the stampings wore out). Another instance of sales didn't meet expectations.